In a production line, the incoming strip stock causes an abrupt load In each stand on the first pass. For the roll drive, this signifies a steep increase of the moment of load. Due to the finite moment of inertia of the drive, made up of the moments of inertia of the motor, the couplings, spindles, gears and rollers, the speed of the drive drops initially. Thereafter, the speed controller brings the speed back to its setpoint. The drop in speed causes a deceleration of the incoming strip as compared to the setpoint speed predetermined in the schedule of passes, resulting in a backward slip of the material. This backward slip is expressed in the formation of a kink in the strip downstream from the stand. This kink may only be of such a magnitude that it can be controlled by the kink control and the kink lifter. The precision of the run-out thickness of the strip is also influenced.
In the case of high demands on the magnitude of the kink, primarily with short load increase times such as occur on the last stands of a production line, a standard PI speed controller is frequently not sufficiently dynamic.